The present invention relates to a combination of a front tire and rear tire for a vehicle having ABS and a vehicle including the combination of the tires.
In recent years, the rate of automobiles having Anti Lock Brake System (ABS) is increasing.
In such Anti Lock Brake Systems, as shown in FIG. 14, the slip rate of tires when ABS is operating is usually adjusted in a range of from about 5% to about 10%, and in order to prevent unstable motions such as spin, a difference is provided between the rear wheels and the front wheels such that the slip rate of the rear tires is smaller than that of the front tires.
An object of the present invention is to provide a combination of a front tire and rear tire having different characteristics which can display maximum braking effect when used with the above-mentioned ABS.
Another object of the present invention is to provide a vehicle having such ABS and equipped with the front tires and rear tires which can display maximum braking effect.
According to one aspect of the present invention, a tire combination for a vehicle having an anti lock brake system comprises
a front tire and a rear tire, each comprising a plurality of tread elements divided by tread grooves,
the rear tire having a tread pattern rigidity Pr and the front tire having a tread pattern rigidity Pf, and
the tread pattern rigidity Pr being more than the tread pattern rigidity Pf.
According to another aspect of the present invention, a vehicle comprises an anti lock brake system and the above-mentioned tire combination.
Here, the tread pattern rigidity P (Pr, Pf) is defined as a quotient of the total of circumferential rigidity of all the tread elements existing in a ground contacting region of the tire which is divided by the area of the ground contacting region. The circumferential rigidity K of each tread element 6 is defined as a quotient of a tangential force F in kgf received at the ground contacting region which is divided by a displacement y in mm at the ground contacting region.   K  =      F    y  
In order to obtain maximum braking effect under the operation of the general ABS, the present inventor studied relationships between frictional coefficient and various factors, and found that it is most important that the tire displays a maximum frictional coefficient under the above-mentioned relatively small slip rate of from 5% to 10%. And as explained above, because a certain difference in slip rate is provided between the front and rear wheels, when the maximum frictional coefficient of the rear tires is less than the maximum frictional coefficient of the front tires, a larger braking force can be obtained.
FIG. 10 shows an exemplary xcexc-s curve, wherein xe2x80x9cxcexcxe2x80x9d on the vertical axis is the frictional coefficient between the tire tread and road surface, and xe2x80x9csxe2x80x9d on the horizontal axis is the slip rate therebetween. In this case, a maximum frictional coefficient occurs ideally at a slip rate of about 7%. In general, however, a maximum frictional coefficient occurs at a relatively higher slip rate. For decreasing the slip rate at which the maximum frictional coefficient occurs, it is preferable to increase the rising angle or inclination angle of a substantially straight portion of the xcexc-s curve rising from the origin of coordinates. Hereinafter, this inclination is called xe2x80x9cxcexc-s stiffnessxe2x80x9d. As a result of inventor""s study, it was also found that the xcexc-s stiffness and the slip rate at the maximum frictional coefficient have a close relationship with a tread pattern rigidity. FIG. 15 shows an exemplary relationship between the pattern rigidity (index) and the slip rate, and an exemplary relationship between the pattern rigidity (index) and the xcexc-s stiffness (index). As shown in this figure, as the pattern rigidity increases, the xcexc-s stiffness increases, and the slip rate at the maximum frictional coefficient decreases. This shows that when the tread pattern rigidity of the rear tire is more than the tread pattern rigidity of the front tire, the slip rate at a maximum frictional coefficient becomes smaller on the rear tire than the front tire, which meets the condition of the ABS. Further, when the xcexc-s stiffness of the rear tire is more than the xcexc-s stiffness of the front tire, the tread pattern rigidity becomes larger on the rear tire than the front tire. This also meets the condition of the ABS.